1. Field of the Invention
The present invention relates to an injector for fuel injection suitable for a vehicle engine such as a diesel engine.
2. Description of the Related Art
In the fields of diesel engines and direct injection type gasoline engines, a common rail fuel injection apparatus is known to be advantageous to pulverization of fuel spray and high pressure injection. One example of the common rail fuel injection apparatuses is disclosed in the pending U.S. patent application Ser. No. 09/136,078 which is owned by the present assignee. In general, the common rail fuel injection apparatus has a common rail (accumulation chamber) for temporarily storing a fuel which has been pressurized by a high pressure pump. The fuel is injected into cylinders of an engine (or combustion chambers) in a predetermined amount at a time from an injector at a predetermined timing.
The injector generally has a plurality of injection openings (fuel spray outlets) at a lower end thereof which are opened and closed by a needle valve moving up and down inside an injector's body (nozzle body). A spring is also received in the injector body to bias the needle valve in a closing direction (downward direction). The needle valve is immersed in the high pressure fuel (i.e., the needle valve would float in a fuel pool if no downward force acts), and a downward spring force is applied to the needle valve so that a lower end of the needle valve is forced to contact a valve seat so as to close the injection openings of the injector. A downward fuel pressure applied on an upper end of the needle valve is controlled by a pressure control chamber (balance chamber). By appropriately leaking a high pressure fuel from the pressure control chamber, the pressure balance relative to the needle valve is lost so that the needle valve is caused to ascend and therefore the injection openings open. This type of injector is disclosed in, for example, Japanese Patent Application Laid-Open Publication Nos. 10-77924, 9-32680 and 9-32682.
The needle valve is received in a hole or bore formed in a nozzle body of the injector such that it can move up and down. The high pressure fuel fed from the common rail flows through a cylindrical clearance between the nozzle body hole and the needle valve and reaches the injection openings of the injector.
Japanese Patent Application Laid Open Publication No. 3-965 also discloses the common rail injector. In this publication, the clearance between the nozzle body hole and the needle valve is made narrower. In other words, this clearance is throttled. With this modification, the fuel pressure downstream of this throttling portion becomes lower than that upstream of the throttling portion when the fuel is injected. Using this pressure difference, a downward force which acts in the valve closing direction is also applied to the needle valve during fuel injection (referred to as "additional downward force"). Therefore, it is possible to close the needle valve at a very high speed upon completion of fuel injection by combination of the spring force and the additional downward force. This improves response and enables use of a relatively weak spring. Accordingly, the valve opening speed is also raised.
However, this arrangement throttles the small clearance in the vicinity of the injection openings (nozzle outlets) to obtain a desired pressure difference. Consequently, it is difficult to obtain a sufficient valve closing force. In general, the larger the area subjected to the fuel pressure, the larger the valve closing force under the same pressure difference. If the small clearance is throttled, a large pressure application area is not obtained. Thus, a large valve closing force does not result.